So you only get an alert for a watched thread the first time it updates. It won't ever alert you again, even if the thread continues to update. Also, it won't update you if you don't read to the very last new post in that thread as well. So you can go back to the thread and read several pages of posts, but if you don't read up to the last page of posts, you won't get alerted for any updates.
If you want to check if you are up to date with a thread you've watched you can check here:
forum.scribblehub.com/watched/threads
If it's bold, you're not up to date and won't get another alert until you are.